Typically, a border of an image-sensing array, such as a Charge Coupled Device (CCD) imager or a CMOS imager, is surrounded with a number of rows and columns of light shielded, or black, pixels. These pixels provide black reference information or black pixel data to stabilize downstream image processing and establish the correct value for black in the output image. Charged Coupled Device Imager (CCD) and CMOS Imager signal processing chains have a need to establish and maintain the black reference in the signal processing chain.
The image-sensing array black pixel data contains a dark current signal, 1/f noise, KTC noise, and shot noise. The black pixel data also contain noise artifacts from imager pixel bright points, pixel dim points and column noise. All of these noise sources require filtering to obtain the correct black value for the image. An appropriate way to remove the noise sources from the black pixel values is to average a large quantity of them.
One component of the dark signal, or black pixel value, from an imager is dark current. CMOS Imager dark current typically doubles for every 10 degrees C. of substrate temperature increase. CCD Imager dark current typically doubles for every 6 degrees C. of substrate temperature increase. As the imager is operated, it heats up, due to its' internal power dissipation.
A “Black Clamp” circuit is typically used to establish the value of the black pixels in an image.
Prior art, Kodak Professional Electronic Cameras, such as the DCS-520, have utilized a black clamp that is a combination of hardware and software. The hardware exists as a digital-to-analog converter and a summing junction in the CCD signal-processing path. The software, executing on a microprocessor, calculates a real time average of the black pixels at the top of the image, it then uses a software look up table to determine the correct value to send to the digital-to-analog converter to establish the correct black correction. This solution requires the availability of the microprocessor and the microprocessor memory. This solution also consumes the power to run the microprocessor and the microprocessor machine cycles which are not available for other functions.
What is needed is a black clamp function that can be implemented on a substrate with the imager, one that uses less power and does not require the microprocessor or microprocessor memory.
What is also needed is a black clamp solution that “tracks” the changes in black level (due to noise and dark current) and removes offset errors in the signal processing and does so in real time.
In an electronic still camera that is capturing a series of sequential images (burst capture mode), the black signal from the imager is increasing as the imager continues to capture pictures. The heating of the imager substrate, due to power dissipation, causes the dark current to increase, increasing the black signal. It is appropriate to correct the black level at the beginning of each image in the series. This requires a real time feedback loop.
Typically, an analog black clamp circuit is utilized prior to a programmable gain stage (PGA) of a camera. Typically, the first stage black clamp takes the form of an integrator whose output is fed to a summing junction, prior to the PGA. This first stage black clamp can also take the form of a programmable offset circuit or a true feedback analog black pixel clamp. (Refer to the TDA9965 ((Philips Semiconductor, The Netherlands), XRD98L61 (EXAR Corp., Fremont, Calif.), and MCM20014 (Motorola, Inc., Denver, Colo.) data sheets). The first stage black clamp does not correct for residual errors that can be caused by the PGA and also does not correct for errors that are caused by the analog-to-digital (A/D) converter circuit appearing at a later stage. What is needed is a circuit that will correct for these errors.
The purpose of the first stage black clamp is to adjust the DC value of the black raw video signal to the “0” value of the Programmable Gain Amplifier (PGA) input. This is necessary because any offset error as input into the PGA is multiplied by the gain of the PGA. For example, a 31 db PGA gain setting corresponds to a 35× multiplication of the amplifier input. If there is a 100 milli-volt input signal (offset error, no black clamp) the amplifier output becomes 3.5 volts. A CMOS Imager typically uses 3.3 volts as its' power source. From this example it can be seen that the PGA will saturate, without using the actual imager video signal. Through this explanation it becomes obvious why the first stage black clamp circuit is necessary.
There are usually residual offset errors at the output of the PGA. These can be due to the PGA itself, and the first stage black clamp not performing “perfectly.” There may be additional offset errors, which accumulate, introduced by each succeeding stage in the signal processing. For example, the analog-to-digital converter may have offset errors.
What is needed is a way to allow for these offset errors and correct for them at an analog-to-digital converter output with a second stage black feedback loop. What is needed is a system for removing these residual errors.
As discussed above, the first stage clamp is required to clamp at the “0” value of the PGA. This is a very tight requirement. What is needed is a way of relieving this performance requirement on the first stage black clamp. Instead of clamping exactly at “0” of the PGA, what is needed is a way of allowing the first stage clamp to get “close enough to 0.”
As discussed above, a software-based solution to the black clamp problem can require unnecessary circuitry, such as memory chips, etc. It also uses valuable processor time. This consumes power and time. What is needed is an electronic circuit solution that is fast, so that it can operate in real time, and compact, so that its will allow for a more compact camera.
What is also needed, and something that is especially valuable when using this invention, is an integrated circuit solution in which the overall design becomes more process variation tolerant, leading to higher device production yield.